1. Field of the Invention
The invention relates to a doppler-radar system for providing a warning if the terrain clearance of an aircraft drops below a predetermined distance at a predetermined slant range. More specifically, the invention utilizes a reference frequency related to the highest doppler frequency measured for a maximum range and the doppler frequency measured for a slant range to make such a determination.
2. Description of the Prior Art
A pilot needs a warning of an obstacle in his flight path twenty to thirty seconds in advance to be able to take corrective action. Accordingly, when traveling at a speed of 600 knots, the pilot must be warned of an obstacle about four miles ahead. Useful information to the pilot would be a warning if the terrain clearance between the aircraft flight path and the obstacle at a four mile slant range drops below a selected predetermined distance beneath the aircraft flight path (hereinafter aircraft safety clearance).
Attempts have been made to determine this information using available doppler-radar systems. One type of doppler-radar obstacle detection system analyzes doppler frequencies to determine if they are varying or constant. This indicates whether there is an obstacle in the flight path but does not provide any terrain clearance information or an indication if the terrain clearance drops below a selected aircraft safety clearance. Another type doppler-radar system uses a known aircraft altitude and speed together with a doppler frequency measured for a ground point to compute the depression angle between the aircraft flight path and ground point. From this information, the slant range to the ground point is computed. This system displays the relative position and size of protuberances for an aircraft ground track on a PPI type display. However, this system has the disadvantages that aircraft altitude and speed must be known and the system cannot provide information as to the size of the object with respect to the height of the aircraft. Both altitude and speed measurements are subject to substantial error.
What is needed is an accurate inexpensive doppler-radar system which can determine, independently of aircraft speed, if terrain clearance at a slant range of four miles drops below a predetermined aircraft safety clearance.